Why the ancient Greeks diluted wine with water?

We are too spoiled the benefits of civilization. Our life today is too sweet. For greater sweetness in all desserts and most natural juices add sugar, although it is available to people 100-200 years ago, and was invented a few centuries ago, whereas in the days of Ancient Greece, and sweeter than honey and the people knew nothing.

the Greeks sweet was the wine, and in those days their wine, they were diluted with clean cold water. Why did they do it?